1. Field of the Invention
The present invention provides a system and method including one or more powered wireless transponders and/or one or more passive wireless transponder devices communicating with a reader/interrogator in which the devices are efficiently inventoried without the risk of eavesdropping.
2. Description of Related Art
A conventional wireless transponder system includes a reader plus a number of wireless transponders, such as radio frequency identification (RFID) transponders. In such a system, the reader needs to identify the transponders within communication range before transmitting commands, either broadcasted to all transponders or to the individual transponders. An anti-collision technique to inventory transponders is described in the ISO 18000-7 standard which is described as follows. First the reader/interrogator sends a wakeup signal to place all transponders in a ready state. An interrogator initiates the tag collection process by sending a Collection command. Transponders receiving a Collection command randomly select a slot in which to respond, but the transponders do not immediately start transmitting. The number of slots in a current collection round is determined by the interrogator command type. Each Collection command requires specific type and amount of data to be transmitted by the tag within a single slot time. The initial window size is fixed and set to 57.3 ms. During the subsequent collision arbitration process, the interrogator dynamically chooses an optimum window size for the next collection round based on the number of collisions in the round. The number of collisions is a function of the number of transponders present within the interrogator communication range that participate in the current collection round. Upon receiving a Collection command, the transponders select a slot in which to respond. The selection is determined by a pseudo-random number generator. When a tag selects a slot_number the tag waits for a pseudo-random time delay equal to a time of slot_number multiplied by slot_delay before it responds. The number of slots is determined by the current window size, indicated through the interrogator collection command type and a tag transmission time. This process is repeated until all transponders respond, specifically until three consecutive collection rounds occur without any tag responses. The technique used in ISO 18000-7 is described in a number of patents, including U.S. Pat. Nos. 5,686,902, 5,640,151 and 6,002,344 and European Patent No. 04607036, all of which cover specific aspects of the ISO 18000-7 technique.
According to the ISO 18000-7 standard and the corresponding patents, when a tag responds, the response packet contains the tag ID, which is permanently assigned to each tag on manufacture. The use of a permanently assigned tag has many disadvantages including that the tag ID is readily available to be read by an eavesdropper, and an unauthorized reader can determine which tagged items are within range. Furthermore, the collection technique can be inefficient in that the window size for collection is initially fixed and the technique requires all transponders to respond after each collection command.
It is desirable to provide an improved anti-eavesdropping anti-collision technique without the risk of eavesdropping.